如何在 Android 中写入 SD 卡上的文件夹？

我使用下面的代码从我的服务器下载一个文件，然后把它写到 SD 卡的根目录，这一切工作正常:

package com.downloader;


import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;


import android.os.Environment;
import android.util.Log;


public class Downloader {


public void DownloadFile(String fileURL, String fileName) {
try {
File root = Environment.getExternalStorageDirectory();
URL u = new URL(fileURL);
HttpURLConnection c = (HttpURLConnection) u.openConnection();
c.setRequestMethod("GET");
c.setDoOutput(true);
c.connect();
FileOutputStream f = new FileOutputStream(new File(root, fileName));


InputStream in = c.getInputStream();


byte[] buffer = new byte[1024];
int len1 = 0;
while ((len1 = in.read(buffer)) > 0) {
f.write(buffer, 0, len1);
}
f.close();
} catch (Exception e) {
Log.d("Downloader", e.getMessage());
}


}
}

但是，使用 Environment.getExternalStorageDirectory();意味着文件将始终写入根 /mnt/sdcard。是否可以指定要写入文件的特定文件夹？

例如: /mnt/sdcard/myapp/downloads

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最佳答案 
File sdCard = Environment.getExternalStorageDirectory();
File dir = new File (sdCard.getAbsolutePath() + "/dir1/dir2");
dir.mkdirs();
File file = new File(dir, "filename");


FileOutputStream f = new FileOutputStream(file);
...
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Add Permission to Android Manifest

Add this WRITE_EXTERNAL_STORAGE permission to your applications manifest. 

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="your.company.package"
android:versionCode="1"
android:versionName="0.1">
<application android:icon="@drawable/icon" android:label="@string/app_name">
<!-- ... -->
</application>
<uses-sdk android:minSdkVersion="7" />
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
</manifest>

Check availability of external storage

You should always check for availability first. A snippet from the official android documentation on external storage.

boolean mExternalStorageAvailable = false;
boolean mExternalStorageWriteable = false;
String state = Environment.getExternalStorageState();


if (Environment.MEDIA_MOUNTED.equals(state)) {
// We can read and write the media
mExternalStorageAvailable = mExternalStorageWriteable = true;
} else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
// We can only read the media
mExternalStorageAvailable = true;
mExternalStorageWriteable = false;
} else {
// Something else is wrong. It may be one of many other states, but all we need
//  to know is we can neither read nor write
mExternalStorageAvailable = mExternalStorageWriteable = false;
}

Use a Filewriter

At last but not least forget about the FileOutputStream and use a FileWriter instead. More information on that class form the FileWriter javadoc. You'll might want to add some more error handling here to inform the user.

// get external storage file reference
FileWriter writer = new FileWriter(getExternalStorageDirectory());
// Writes the content to the file
writer.write("This\n is\n an\n example\n");
writer.flush();
writer.close();
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Found the answer here - http://mytechead.wordpress.com/2014/01/30/android-create-a-file-and-write-to-external-storage/

It says, 

/**


* Method to check if user has permissions to write on external storage or not


*/


public static boolean canWriteOnExternalStorage() {
// get the state of your external storage
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
// if storage is mounted return true
Log.v("sTag", "Yes, can write to external storage.");
return true;
}
return false;
}

and then let’s use this code to actually write to the external storage:

// get the path to sdcard
File sdcard = Environment.getExternalStorageDirectory();
// to this path add a new directory path
File dir = new File(sdcard.getAbsolutePath() + "/your-dir-name/");
// create this directory if not already created
dir.mkdir();
// create the file in which we will write the contents
File file = new File(dir, "My-File-Name.txt");
FileOutputStream os = outStream = new FileOutputStream(file);
String data = "This is the content of my file";
os.write(data.getBytes());
os.close();

And this is it. If now you visit your /sdcard/your-dir-name/ folder you will see a file named - My-File-Name.txt with the content as specified in the code.

PS:- You need the following permission -

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
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In order to download a file to Download or Music Folder In SDCard 

File downlodDir = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS);// or DIRECTORY_PICTURES

And dont forget to add these permission in manifest

 <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

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